The crushed/milled ore from which the uranium has been chemically extracted contains uranium’s radioactive decay products, such as thorium-230 and radium-226. These products will continue to be radioactive for thousands of years. Thorium-230 has a half-life of 77,000 years. Even once the uranium has been removed from the ore, the remaining tailings retain 85% of the radioactivity of the original ore. This waste, in the form of slurry, is pumped to a tailings management area, which is usually a natural or man-made pond, where the solids fall to the bottom.
The tailings pond is designed to hold the solids, which are covered by several metres of water. The water is an effective barrier that isolates the radiation. However, the slurry containing the solid radioactive and toxic elements is expected to sink to the bottom and remain in the sediment. The water will contain dissolved radioactive elements and other toxic metals. It could be poisonous to animals drinking or swimming in the water, or to birds landing on the water. Note that if the sediment is disturbed, the water quality will deteriorate further. If the water evaporates, its effectiveness as a radiation barrier is compromised. Hence the need to remove the tailings or manage tailings ponds over the long-term (tens of thousands of years).
A growing practice is to repurpose a natural lake into a tailings pond. This is of benefit to the mining company since engineering a man-made tailings pond is an expensive undertaking. Although current regulations, as a general rule, protect water frequented by fish, exceptions can and often are made by adding the water body to a list of exempted areas under the federal Metals Mining and Effluent Regulations. This would allow an existing lake to be used as a tailings pond. The lake would no longer remain a viable environment for aquatic life.